Measuring apparatus, measuring method, testing apparatus, testing method, and electronic device

ABSTRACT

There is provided a measuring apparatus for measuring a signal-under-test, having a comparator for sequentially comparing voltage values of the signal-under-test with a threshold voltage value fed thereto at timing of strobe signals sequentially fed thereto, a strobe timing generator for sequentially generating the strobe signals placed almost at equal time intervals, a capture memory for storing the comparison result of the comparator and a digital signal processing section for calculating jitter of the signal-under-test based on the comparison result stored in the capture memory.

BACKGROUND

1. Field of the Invention

The present invention relates to a measuring apparatus and a measuringmethod for measuring a signal-under-test, a testing apparatus and atesting method for testing a device-under-test and an electronic device.More specifically, the invention relates to a measuring apparatus, ameasuring method, a testing apparatus and a testing method for measuringjitters in the signal-under-test outputted out of the device-under-test.

2. Related Art

There has been known a test for measuring jitter in a signal-under-testoutputted out of an electronic device such as a semiconductor circuit asan item for testing the electronic device. For example, jitter of suchsignal-under-test is measured by a time interval analyzer, anoscilloscope or the like by inputting the signal-under-test thereto. Thetime interval analyzer or the like allows such jitter to be calculatedby measuring phase errors of edges in the signal-under-test for example.

Still more, there has been known a functional test for judging whetheror not a pattern of a signal-under-test outputted out of an electronicdevice coincides with a pattern of an expected values as an item fortesting the electronic device. In this test, a testing apparatus detectsa data pattern of the signal-under-test by comparing a voltage values ofthe signal-under-test outputted out of the electronic device withthreshold voltage when a predetermined test pattern is inputted to theelectronic device. Then, it judges whether or not the data patterncoincides with the pattern of the expected values.

It has been thus necessary to prepare the apparatus for measuring jitterand the apparatus for testing functionality of the device in order tocarry out the jitter test in addition to functional tests as describedabove. Therefore, it has been costly to carry out the jitter test.

Still more, the apparatus of functional test compares the voltage valueof the signal-under-test with the threshold voltage at preset timing.Therefore, it can detect the edge position or timing, over which thedata pattern of the signal-under-test transits bit by bit, by shiftingthe comparison timing. It is then conceivable to be able to measurejitter by utilizing this function, i.e., by using the apparatus forperforming functional testing.

However, the conventional apparatus for functional testing sets and usessampling timings based on a test rate synchronized with the operatingperiod of the signal-under-test. Therefore, for each test rate it isnecessary to set phase of the sampling timing in order to graduallyshift the relative phase of the sampling timing with respect to thesignal-under-test within each test rate. It has been thus necessary tocarry out the cumbersome timing setting in order to carry out the jittertest, and it has taken a significantly long time for testing. Stillmore, its measuring accuracy is insufficient and is not suitable fortests because its timing is shifted in the relative phase fashion.

When jitter is measured by using an oscilloscope or the like, thesignal-under-test to be inputted therein contains amplitude noisecomponents in addition to the timing noise component. Therefore, it hasbeen difficult to accurately measure only the timing noise of thesignal-under-test.

Accordingly, it is an advantage of the invention to provide a measuringapparatus, a measuring method, a testing apparatus, a testing method andan electronic device, which are capable of solving the above-mentionedproblem. This advantage may be achieved through the combination offeatures described in independent claims of the invention. Dependentclaims thereof specify preferable embodiments of the invention.

SUMMARY

In order to solve the above-mentioned problems, according to a firstaspect of the invention, there is provided a measuring apparatus formeasuring a signal-under-test, having a comparator for sequentiallycomparing voltage values of the signal-under-test with a thresholdvoltage value fed thereto at timing of strobe signals sequentially fedthereto, a strobe timing generator for sequentially generating thestrobe signals disposed almost at equal time intervals, a capture memoryfor storing the comparison result of the comparator and a digital signalprocessing section for calculating jitter of the signal-under-test basedon the comparison result stored in the capture memory.

The strobe timing generator may sequentially generate the strobe signalsat the cycle different from that of the signal-under test by apredetermined value. The strobe timing generator may sequentiallygenerate the strobe signals at a cycle larger than half of that of thesignal-under-test.

The strobe timing generator may sequentially generate the strobe signalssuch that the difference between the cycle of the strobe signal and thatof the signal-under-test is an equivalent sampling interval dependent ona jitter value to be measured.

The strobe timing generator may provide the jitter value to be measuredand sequentially generate the strobe signals such that the differencebetween the cycle of the strobe signal and that of the signal-under-testis less than N times of jitter value, where N is a positive integer.

The strobe timing generator may sequentially generate the strobe signalssuch that the difference between the cycle of the strobe signal and thatof the signal-under-test is a value dependent on a time resolution forcalculating jitter.

The strobe timing generator may sequentially generate the strobe signalssuch that the time resolution for calculating jitter is provided and thedifference between the cycle of the strobe signal and that of thesignal-under-test is less than the time resolution.

The digital signal processing section may divide the cycle of thesignal-under-test by the difference between the cycle of the strobesignal and that of the signal-under-test, retrieve continuous data withthe number of points which is integral multiple of the division resultand calculate the jitter based on the retrieved data.

When the number of points of the captured data is power-of-two, thedigital data processing section calculate jitter based on the resultobtained by performing a fast Fourier Transform on the retrieved data.Alternatively, when the number of points of the captured data is notpower-of-two, the digital data processing section calculate jitter basedon the result obtained with the mixed-radix algorithm.

The measuring apparatus may further include a digital signal convertingsection for generating a digital signal in which each voltage value ofthe signal-under-test is converted into a digital value whose absolutevalue falls within a range smaller than n (where, n is a real number)based on the comparison result stored in the capture memory, and thedigital signal processing section may calculate the jitter of thesignal-under-test based on the digital signal.

The comparator may output the comparison results different from eachother depending on whether or not the voltage value of thesignal-under-test is greater than the threshold voltage value. Thedigital value converting section may convert the comparison resultrepresenting that the voltage value of the signal-under-test is greaterthan the threshold voltage value into a digital value of 1 and mayconvert the comparison result representing that the voltage value of thesignal-under-test is smaller than the threshold voltage value into adigital value of 0.

The comparator may receive first one of the threshold voltage and secondone of the threshold voltage whose voltage value is lower than the firstthreshold voltage and may output the comparison results different fromeach other depending on whether or not the voltage value of thesignal-under-test is greater than the first threshold voltage, whetheror not the voltage value of the signal-under-test is smaller than thefirst threshold voltage and greater than the second threshold voltage orwhether the voltage value of the signal-under-test is smaller than thesecond threshold voltage. The digital signal converting section mayconvert the comparison result indicating that the voltage value of thesignal-under-test is greater than the first threshold voltage value intoa digital value 1, may convert the comparison result indicating that thevoltage value of the signal-under-test is smaller than the firstthreshold voltage value and is greater than the second threshold voltagevalue into a digital value 0 and may convert the comparison resultindicating that the voltage value of the signal-under-test is smallerthan the second threshold voltage value into a digital value −1.

The comparator may receive three or more different threshold voltagevalues and may output comparison results different from each otherdepending on a voltage range specified by two neighboring thresholdvoltages to which the voltage value of the signal-under-test belongs.

The strobe timing generator may generate the strobe signals disposedalmost at equal time intervals independently of operation periods of themeasuring apparatus. The strobe timing generator may generate one of thestrobe signals per operation period of the measuring apparatus. Thestrobe timing generator may generate the plurality of strobe signals peroperation period of the measuring apparatus.

The digital signal processing section may have a band limiting sectionfor passing a frequency component to be measured of the digital signaland a phase distortion estimating section for calculating phase noise ofthe digital signal outputted out of the band limiting section.

The band limiting section may convert the digital signal into ananalytic signal, and the phase distortion estimating section may have aninstantaneous phase estimating section for generating an instantaneousphase signal representing the instantaneous phase of thesignal-under-test and a linear phase removing section for removing alinear component of the instantaneous phase signal to calculate phasenoise of the signal-under-test.

The phase distortion estimating section may have a zero-crossing timingestimating section for estimating zero-crossing timing series of thesignal-under-test based on the digital signals outputted out of the bandlimiting section and a linear phase removing section for removing alinear component of the zero-crossing timing series to calculate phasenoise of the signal-under-test.

The measuring apparatus may further include a filter for passing thefrequency component to be measured of the signal-under-test to input tothe comparator. The filter may pass a frequency component of frequencyband containing no carrier frequency of the signal-under-test amongfrequency components of the signal-under-test.

The measuring apparatus may further include a plurality of comparatorsarranged in parallel and an input section for inputting thesignal-under-test into each one of the plurality of comparators inparallel, and the strobe timing generator may input the strobe signalswhose phases are different to the respective comparators and the capturememory may align and store the comparison results in the plurality ofcomparators in accordance to phases of the corresponding strobe signals.

The strobe timing generator may generate the strobe signals to beinputted to the comparators based on phases of a triggering signalsynchronized with the signal-under-test. The measuring apparatus mayfurther include a clock regenerator for generating a recovered clocksynchronized with the signal-under-test and for inputting the recoveredclock to the strobe timing generator as the triggering signal.

The strobe timing generator may sequentially input the plurality ofstrobe signals whose phases are different with respect to the triggeringsignal to the respective comparators and the capture memory may alignand store the comparison results outputted out of the comparatorscorresponding to the respective triggering signals in accordance to thephases of the corresponding strobe signals.

According to a second aspect of the invention, there is provided atesting apparatus for testing a device-under-test, having a measuringapparatus for measuring jitter of a signal-under-test outputted out ofthe device-under-test, and a jitter judging section for judging whetheror not the device-under-test is defect-free based on the jitter measuredby the measuring apparatus, wherein the measuring apparatus has acomparator for sequentially comparing voltage values of thesignal-under-test with a threshold voltage value given thereto at timingof strobe signals sequentially fed thereto, a strobe timing generatorfor sequentially generating the strobe signals disposed almost at equaltime intervals, a capture memory for storing comparison results of thecomparator and a digital signal processing section for calculating thejitter of the signal-under-test based on the comparison results storedin the capture memory.

The testing apparatus may further include a logic judging section forjudging whether or not a data pattern of the signal-under-testdetermined by the comparison results stored in the capture memorycoincides with a preset expected value pattern.

The strobe timing generator may sequentially generate the strobe signalsat a cycle different from that of the signal-under test by apredetermined value.

The testing apparatus may further includes a driver that operatesaccording to a predetermined test rate, for causing the device undertest to output the signal-under-test at the cycle according to the testrate. The strobe timing generator may sequentially generate the strobesignals at a cycle larger than the test rate by a predetermined value.

The testing apparatus may further include a logic judging section forjudging whether or not a data pattern of the signal-under-testdetermined by the comparison results stored in the capture memorycoincides with a preset expected value pattern.

According to a third aspect of the invention, there is provided ameasuring method for measuring a signal-under-test having apredetermined period, having a comparing step of sequentially comparingvoltage values of the signal-under-test with a given threshold voltagevalue at timing of strobe signals sequentially fed thereto, a strobetiming generating step of sequentially generating the strobe signalsdisposed almost at equal time intervals, a storing step of storingcomparison results of the comparator and a digital signal processingstep of calculating the jitter of the signal-under-test based on thecomparison results stored in the storing step.

According to a fourth aspect of the invention, there is provided atesting method for testing a device-under-test, having a measuring stepof measuring jitter of a signal-under-test outputted out of thedevice-under-test and a jitter judging step of judging whether or notthe device-under-test is defect-free based on the jitter measured in themeasuring step, wherein the measuring step includes a comparing step ofsequentially comparing voltage values of the signal-under-test with agiven threshold voltage value at timing of strobe signals sequentiallyfed, a strobe timing generating step of sequentially generating thestrobe signals disposed almost at equal time intervals, a storing stepof storing comparison results of the comparator and a digital signalprocessing step of calculating the jitter of the signal-under-test basedon the comparison results stored in the storing step.

According to a fifth aspect of the invention, there is provided ameasuring apparatus for measuring a signal-under-test having apredetermined period, having a comparator for sequentially comparing avoltage value of the signal-under-test, a first threshold voltage valueand a second threshold voltage value given thereto at timing of strobesignals sequentially given thereto to output comparison results of threevalues, a capture memory for storing the comparison results of thecomparator and a digital signal processing section for calculatingjitter of the signal-under-test based on the comparison result stored inthe capture memory.

The digital signal processing section may have a Hilbert transformationpair generating section for transforming the comparison result into ananalytic signal, an instantaneous phase estimating section forgenerating an instantaneous phase signal representing an instantaneousphase of the signal-under-test based on the analytic signal and a linearphase removing section for removing a linear phase of the instantaneousphase signal to present phase noise of the signal-under-test.

According to a sixth aspect of the invention, there is provided anelectronic device for outputting a signal-under-test, having anoperation circuit for generating the signal-under-test and a measuringapparatus for measuring the signal-under-test, and the measuringapparatus has a comparator for sequentially comparing voltage values ofthe signal-under-test with a threshold voltage value given thereto attiming of strobe signals sequentially given thereto and a capture memoryfor storing the comparison result of the comparator.

The electronic may further include a strobe timing generator forsequentially generating strobe signals arranged almost at equal timeintervals.

The strobe timing generator may sequentially generate the strobe signalsat a cycle different from that of the signal-under-test by apredetermined value.

According to a seventh aspect of the invention, there is provided ameasuring apparatus for measuring a signal-under-test. The measuringapparatus includes: a first comparator for sequentially comparingvoltage values of a first signal-under-test with a threshold voltagevalue fed thereto at timing of strobe signals sequentially fed theretoat a timing at which the strobe signals are sequentially fed thereto; asecond comparator for sequentially comparing voltage values of a secondsignal-under-test with a threshold voltage value fed thereto atapproximately the same time; a strobe timing generator for sequentiallygenerating the strobe signals placed almost at equal time intervals; acapture memory for storing the comparison result of the comparator; anda digital signal processing section for calculating the instantaneousphase for each of the first signal-under-test and the secondsignal-under-test based on the comparison result stored in the capturememory and calculating a deterministic skew between the firstsignal-under-test and the second signal-under-test based on eachinstantaneous phase.

According to an eighth aspect of the invention, there is provided ameasuring apparatus for measuring a signal-under-test. The measuringapparatus includes: a first comparator for sequentially comparingvoltage values of a first signal-under-test with a threshold voltagevalue fed thereto at timing of strobe signals sequentially fed theretoat a timing at which the strobe signals sequentially fed thereto; asecond comparator for sequentially comparing voltage values of a secondsignal-under-test with a threshold voltage value fed thereto atapproximately the same time; a strobe timing generator for sequentiallygenerating the strobe signals placed almost at equal time intervals; acapture memory for storing the comparison result of the comparator; anda digital signal processing section for calculating the instantaneousphase for each of the first signal-under-test and the secondsignal-under-test based on the comparison result stored in the capturememory, calculating the instantaneous phase noise for each of the firstsignal-under-test and the second signal-under-test based on eachinstantaneous phase and calculating a random skew between the firstsignal-under-test and the second signal-under-test based on eachinstantaneous phase noise. Therefore, the timing for each edge of thesignal-under-test may be changed within two-three times of the jittersignal around the ideal

It is noted that the summary of the invention described above does notnecessarily describe all necessary features of the invention. Theinvention may also be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing one exemplary configuration of a testingapparatus according one embodiment of the invention.

FIG. 2 is a chart showing exemplary strobe signals generated by a strobetiming generator.

FIGS. 3A, 3B and 3C are diagrams showing exemplary configurations of acomparator.

FIG. 4 shows one exemplary operation of the measuring apparatus when thecomparator shown in FIG. 3A is used.

FIGS. 5A and 5B are diagrams showing exemplary configurations of adigital signal processing section.

FIGS. 6A and 6B are graphs showing exemplary operations of a linearphase removing section.

FIG. 7 is a table showing jitter values actually measured by the testingapparatus as compared to jitter values actually measured by aconventional jitter measuring method.

FIGS. 8A and 8B show exemplary configurations of a band limitingsection.

FIG. 9 is a graph showing one exemplary frequency band passed through afilter.

FIG. 10 shows another exemplary configuration of the measuringapparatus.

FIG. 11 is a chart showing one exemplary operation of the comparator andthe strobe timing generator.

FIG. 12 is a diagram showing another exemplary configuration of themeasuring apparatus.

FIG. 13 is a diagram showing another exemplary configuration of thecomparator.

FIG. 14 is a chart showing one exemplary operation of the comparator andthe strobe timing generator shown in FIG. 13.

FIGS. 15 and 16 are flowcharts showing one exemplary method forcorrecting errors of sampling timing.

FIG. 17 is a diagram showing another exemplary configuration of thetesting apparatus.

FIG. 18 is a diagram showing one exemplary configuration of anelectronic device according to another embodiment of the invention.

FIG. 19 is a diagram showing a strobe signal generated by the strobetiming generator 30.

FIG. 20 is a diagram explaining an example of operation of the digitalsignal processing section 60.

FIG. 21 is a diagram explaining an example of operation of the digitalsignal processing section 60.

FIG. 22 is a diagram an example of instantaneous phase noise Δφ(t)obtained by sampling a signal-under-test with a strobe signal having acycle resolution (Δ) from the cycle of the signal-under-test.

FIG. 23 is a diagram illustrating the measuring bandwidth dependency ofthe jitter value calculated for each cycle resolution (Δ).

FIG. 24 is a diagram showing an example of measurement error of thejitter value calculated for the cycle resolution (Δ) between each cycle.

FIG. 25 is a diagram showing an example of configuration of a patterngenerator 65 included in a testing apparatus 100.

FIG. 26 shows an example of instantaneous phase φ(t), linear phase andinstantaneous phase noise Δφ(t) of the signal-under-test described inFIG. 21.

FIG. 27 is a diagram showing a comparison between the result ofmeasuring jitter by the testing apparatus 100 and the result ofmeasuring jitter by the conventional jitter measuring apparatus.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention will now be described based on preferred embodiments,which do not intend to limit the scope of the invention, but exemplifythe invention. All of the features and the combinations thereofdescribed in the embodiments are not necessarily essential to theinvention.

FIG. 1 is a diagram showing one exemplary configuration of a testingapparatus 100 according one embodiment of the invention. The testingapparatus 100 is an apparatus for testing a device-under-test (DUT) 200such as a semiconductor circuit and has a measuring apparatus 10 and ajudging section 70. The measuring apparatus 10 measures jitter in asignal-under-test outputted out of the DUT 200. Here, thesignal-under-test is a signal having a predetermined period. Thesignal-under-test may be a clock signal or a data signal for example.The measuring apparatus 10 may also measure timing jitter of thesignal-under-test.

The judging section 70 judges the pass/fail result for the device undertest based on the jitter value in the signal-under-test measured by themeasuring apparatus 10. For example, the judging section 70 may judgethe pass/fail result for the device under test 200 based on whether thevalue of timing jitter in the signal-under-test is more than apredetermined threshold value. The threshold value may be defined by thespecifications of the required device under test 200.

The measuring apparatus 10 has a comparator 20, a strobe timinggenerator 30, a capture memory 40, a digital signal converting section50 and a digital signal processing section 60. The comparator 20compares voltage values of the signal-under-test with a thresholdvoltage value fed thereto at timing of strobe signals fed sequentiallythereto.

The strobe timing generator 30 sequentially generates the strobe signalsat almost equally-spaced time intervals. The strobe timing generator 30may sequentially generate the strobe signals in synchronism with theperiod of the signal-under-test.

Still more, the strobe timing generator 30 may sequentially generate thestrobe signals independently from the period of the signal-under-test.The strobe timing generator 30 may also sequentially generate the strobesignals in synchronism with a period different from the period of thesignal-under-test.

The capture memory 40 stores the comparison result outputted out of thecomparator 20. For example, the capture memory 40 aligns and stores thecomparison results outputted out of the comparator 20 with respect tothe phase of the corresponding strobe signals.

The digital signal processing section 60 calculates jitter of thesignal-under-test based on the comparison results stored in the capturememory 40. The digital signal processing section 60 may calculate thejitter of the signal-under-test by methods described later in connectionwith FIGS. 5A and 5B for example. The digital signal processing section60 may also calculate the jitter of the signal-under-test by means ofother known technologies.

It is preferable to input data corresponding to the signal processingmethod of the digital signal processing section 60 to the digital signalprocessing section 60. When the digital signal processing section 60calculates the jitter of the signal-under-test based on zero-crossingpoints or the like of the signal-under-test for example, it ispreferable to input a signal whose absolute value of amplitude presentsa discrete value in a range smaller than n (where, n is a real number).

The measuring apparatus 10 of this example converts the comparisonresults stored in the capture memory 40 into a digital signal to beinputted to the digital signal processing section 60. For example, thedigital signal converting section 50 generates the digital signal inwhich the respective voltage values of the signal-under-test areconverted into digital values in a range in which their absolute valuesare smaller than n (where n is a real number). For example, the digitalsignal converting section 50 may convert the comparison results into thedigital values approximately from 1 to −1.

A case when the comparator 20 compares the voltage values of thesignal-under-test with one threshold voltage at timing of the respectivestrobe signals and outputs a logical value H or logical value L as thecomparison result will be explained as an example. For this example, thedigital signal converting section 50 outputs a digital signal in whichthe logic H is converted into a digital value 1 and the logic L isconverted into a digital value −1. When the comparator 20 output thecomparison results of three values, the digital signal convertingsection 50 converts the respective comparison results into digitalvalues 1, 0 and −1 corresponding to the logical values of the comparisonresults.

Such signal conversion facilitates signal processing in the digitalsignal processing section 60.

FIG. 2 is a chart showing exemplary strobe signals generated by thestrobe timing generator 30. This example will be explained by settingthe period of the signal-under-test as T. The strobe timing generator 30sequentially generates the strobe signals at almost equally-spaced timeintervals synchronously or asynchronously with the period T of thesignal-under-test as described above.

Generally, the testing apparatus 100 operates per cycle (T0, T1, T2, . .. ) corresponding to operating period (test rate) synchronized with theperiod T of the signal-under-test. The strobe timing generator 30 maygenerate one strobe signal or a plurality of strobe signals per cyclecorresponding to the test rate as shown by the strobe signals (1) and(2) in FIG. 2. Still more, the strobe timing generator 30 may generatethe strobe signals asynchronously from the test rate as shown by thestrobe signals (3) in FIG. 2. At this time, a number of strobe signalsgenerated with respect to each cycle is determined by the period and thetest rate, by which the strobe timing generator 30 generates the strobesignals. For example, the strobe timing generator 30 may be anoscillation circuit that operates independently from the operationperiod of the testing apparatus 100.

Still more, the period T of the signal-under-test may or may notcoincide with the test rate of the testing apparatus 100. It ispreferable for the period T of the signal-under-test to coincide withthe test rate when the testing apparatus 100 also performs a functionaltest described later.

It is possible, by setting the intervals Ts of the strobe signalsgenerated by the strobe timing generator 30 as described above, tosequentially generate strobe signals whose phase is gradually shiftedwith respect to the phase of the signal-under-test Also, as the strobesignals in which strobes are placed almost at equal time intervals, thestrobe timing generator 30 may generate either one of (1) the strobesignals in which one strobe is placed per test rate, (2) the strobesignals in which a plurality of strobes is placed per test rate and (3)the strobe signals in which strobes are placed independently from thetest rate.

Although the case in which the test rate of the testing apparatus 100 isequal with the period T of the signal-under-test has been describedabove, the test rate of the invention is not necessary to be equal tothe period T of the signal-under-test when no function test is performedand can be set independently from the period T.

FIGS. 3A, 3B and 3C are diagrams showing exemplary configurations of thecomparator 20. The comparator 20 shown in FIG. 3A is supplied with afirst threshold voltage VOH and a second threshold voltage VOL andoutputs a comparison result of three values. A case when the secondthreshold voltage VOL is smaller than the first threshold voltage VOHwill be explained in this example. The comparator 20 outputs eachdifferent comparison result in each case when the voltage value of thesignal-under-test is larger than the first threshold voltage VOH, whenthe voltage value of the signal-under-test is less than the firstthreshold voltage VOH and is larger than the second threshold voltageVOL and when the voltage value of the signal-under-test is less than thesecond threshold voltage VOL.

The comparator 20 has a first comparator 22-1 and a second comparator22-2. The signal-under-test is split into two and fed respectively tothe comparators 22-1 and 22-2. The strobe timing generator 30 feeds thestrobe signals representing almost same timing to the comparators 22-1and 22-2.

The first comparator 22-1 compares a voltage value of thesignal-under-test with the first threshold voltage VOH per strobe signalfed thereto. The first comparator 22-1 outputs a logical valuerepresenting High when the voltage value of the signal-under-test isgreater than the first threshold voltage VOH and outputs a logical valuerepresenting Low when the voltage value of the signal-under-test issmaller than the first threshold voltage VOH for example.

The second comparator 22-2 compares the voltage value of thesignal-under-test with the second threshold voltage VOL per strobesignal fed thereto. The second comparator 22-2 outputs a logical valuerepresenting High when the voltage value of the signal-under-test isgreater than the second threshold voltage VOL and outputs a logicalvalue representing Low when the voltage value of the signal-under-testis smaller than the second threshold voltage VOL for example.

The comparator 20 outputs a set of the logical values outputted out ofthe comparators 22-1 and 22-2 as its comparison result. That is, whenthe logical value outputted out of the first comparator 22-1 is supposedto be M and the logical value outputted out of the second comparator22-2 is supposed to be N, the comparator 20 outputs the comparisonresult of three values of (M, N)=(High, High), (Low, High) and (Low,Low) corresponding to the voltage value of the signal-under-test.

In this case, the digital signal converting section 50 converts therespective comparison results (High, High), (Low, High) and (Low, Low)into digital values of 1, 0 and −1, respectively, for example.

The comparator 20 shown in FIG. 3B outputs different comparison resultsdepending on whether or not the voltage value of the signal-under-testis greater than a threshold voltage value VT fed thereto. That is, thecomparator 20 of this example outputs the binary comparison results. Thecomparator 20 has a comparator 22 to which the threshold voltage valueVT and the signal-under-test are inputted. The comparator 22 comparesthe voltage value of the signal-under-test with the threshold voltagevalue VT at the timings of the strobe signals fed from the strobe timinggenerator 30. When the voltage value of the signal-under-test is greaterthan the threshold voltage value VT for example, the comparator 22outputs the logical value representing High and when the voltage valueof the signal-under-test is smaller than the threshold voltage value VT,the comparator 22 outputs the logical value representing Low. Thecomparator 20 outputs the logical value outputted out of the comparator22 as a comparison result.

In this case, the digital signal converting section 50 converts therespective comparison results High and Low into digital values of 1 and−1, respectively, for example.

The comparator 20 shown in FIG. 3C has a plurality of comparators 22.Threshold voltages VT1, VT2, . . . different from each other are fed tothe respective comparators 22. Still more, the signal-under-test issplit and is fed into the respective comparators 22. The strobe timinggenerator 30 feeds the strobe signals of almost same timing to therespective comparators 22.

The respective comparators 22 compare the voltage value of thesignal-under-test with the corresponding threshold voltage VTx at thetimings of the strobe signals fed thereto. The operation of eachcomparator 22 is the same with the comparator 22 shown in FIG. 3B. Thecomparator 20 outputs sets of the logical values outputted out of therespective comparators 22 as comparison results.

That is, the comparator 20 of this example outputs the comparisonresults different from each other depending on that the voltage value ofthe signal-under-test belongs to which one of respective voltage rangespecified by the neighboring two threshold voltages among the three ormore types of threshold voltages VT fed thereto.

For example, the digital signal converting section 50 converts thecomparison result in which the logical values outputted out of all ofthe comparators 22 represent High into a digital value of 1 and convertsthe comparison result in which the logical values outputted out of allof the comparators 22 represents Low into the digital value of −1. Thedigital signal converting section 50 also converts the other comparisonresult into a predetermined digital value between 1 and −1 correspondingits logical value.

Preferably, each threshold voltage fed to the comparator 20 explained inFIGS. 3A through 3C is variable. For example, the measuring apparatus 10may control each threshold voltage based on information of amplitudelevel to be measured of the signal-under-test.

FIG. 4 shows one exemplary operation of the measuring apparatus 10 whenthe comparator 20 shown in FIG. 3A is used. The signal-under-test asillustrated in FIG. 4 is inputted to the measuring apparatus 10. Theinput signal contains timing noise, i.e., jitter along the time axis,and amplitude noise along the amplitude axis. The jitter of the timingnoise is dominant over the edge portions of the signal-under-test andthe amplitude noise is dominant over the stationary portions of thesignal-under-test.

As shown in FIG. 4, the vertical eye opening of the signal-under-testdecreases due to the amplitude noise and the horizontal eye openingthereof decreases due to the timing noise. Ideally, the horizontal eyeopening of the signal-under-test is affected only by the timing noise.However, the amplitude noise also affects the horizontal eye opening dueto a kind of AM-to-PM conversion. Consequently, the amplitude noise hasa relatively high probability of being converted into timing noise.

Then, it is desired to measure timing jitter by removing the influenceof the amplitude noise.

However, the measuring apparatus 10 of this example converts the voltagevalue of the signal-under-test that is greater than the first thresholdvoltage VOH into the digital value of 1 and converts the voltage valueof the signal-under-test smaller than the second threshold voltage VOLinto the digital value of −1. Thereby, the influence of the amplitudenoise can be removed automatically. Then, the measuring apparatus 10converts the voltage value of the signal-under-test smaller than thefirst threshold voltage VOH and greater than the second thresholdvoltage VOL into the digital value of 0. The timing when those digitalvalues are detected may be determined only by the timing noise.Therefore, it is possible to accurately measure the timing noise byremoving the influence of the amplitude noise based on the comparisonresults of the comparator 20.

Still more, the strobe signals inputted to the comparator 20 arearranged almost at equal intervals independently of the stationaryperiod of the signal-under-test. Therefore, it enables the measurementto be carried out by excluding the time dependency of the timing noise.Preferably, frequency by which the strobe signals are inputted to thecomparator 20 is larger than Nyquist frequency. For example, four ormore strobe signals may be disposed in each period of thesignal-under-test.

FIGS. 5A and 5B are diagrams showing exemplary configurations of thedigital signal processing section 60. The digital signal processingsection 60 shown in FIG. 5A has a band limiting section 62 and a phasedistortion estimating section 64. The band limiting section 62 passesfrequency components to be measured of the discrete signal. Still more,the band limiting section 62 of this example converts the discretesignal into an analytic signal. The band limiting section 62 maygenerate the analytic signal by generating a Hilbert transformationpair.

The digital signal converting section 50 converts the comparison resultsoutputted out of the comparator 20 into the discrete signalsrepresenting the digital values of 1, 0 and −1 for example as describedabove. Therefore, the digital signal converting section 50 can generatea signal corresponding to that discrete signal and may generate ananalytic signal cos(2πft)+j sin(2πft) for example. The influence of theamplitude noise of the signal-under-test has been removed out of theanalytic signal as described above.

The phase distortion estimating section 64 calculates phase noise of thediscrete signal outputted out of the band limiting section 62. The phasedistortion estimating section 64 of this example has an instantaneousphase estimating section 66 and a linear phase removing section 68.

Based on the analytic signal outputted out of the band limiting section62, the instantaneous phase estimating section 66 generates aninstantaneous phase signal presenting instantaneous phase of thediscrete signal. The instantaneous phase of the discrete signal may befound from inverse-tangent (arctangent) of the ratio of a real part andan imaginary part of the analytic signal.

The linear phase removing section 68 calculates phase noise of thesignal-under-test by removing the linear component of the instantaneousphase signal. For example, the linear phase removing section 68 maycalculate the linear component of the instantaneous phase signal basedon the period of the signal-under-test or may calculate the linearcomponent in which the instantaneous phase signal is approximated by astraight line. The linear component is the instantaneous phase of thejitter-free signal-under-test. The linear phase removing section 68 mayalso calculate the linear component based on a measured average periodof the signal-under-test. A difference of the instantaneous phase signalwith respect to the linear component represents the phase noise of thesignal-under-test at each timing.

The digital signal processing section 60 shown in FIG. 5B has a bandlimiting section 62 and a phase distortion estimating section 64. Theband limiting section 62 passes frequency components to be measured ofthe discrete signal.

The phase distortion estimating section 64 has a zero-crossing timingestimating section 72 and a linear phase removing section 68. Thezero-crossing timing estimating section 72 estimates a zero-crossingtiming series of the signal-under-test based on the discrete signaloutputted out of the band limiting section 62. The zero-crossing timingseries is data that sequentially represents timing when the discretesignal presents the digital value “0”.

The linear phase removing section 68 removes the linear component of thezero-crossing timing series and calculates the phase noise of thesignal-under-test. The linear component may be calculated by the samemethod performed by the linear phase removing section 68 shown in FIG.5A.

FIGS. 6A and 6B are graphs showing exemplary operations of the linearphase removing section 68. FIG. 6A shows the instantaneous phase of thediscrete signal, where an axis of abscissa represents time t and an axisof ordinate represents instantaneous phase φ(t). It is possible to findthe phase error of the discrete signal by finding the difference betweenthe instantaneous phase and its linear component as described above.

FIG. 6B is a graph obtained by plotting each zero-crossing timing, wherean axis of abscissa represents time t and an axis of ordinate representszero-crossing timing. It is then possible to find the phase error ofeach zero-crossing point, i.e., the phase error of the edge of thesignal-under-test, by finding the difference from the linear componentat each zero-crossing point as described above.

FIG. 7 is a table showing jitter values actually measured by the testingapparatus 100 as compared to jitter values actually measured by aconventional jitter measuring method. According to the conventionaljitter measuring method shown in FIG. 7, an ADC of 8 bits converts thesignal-under-test into discrete signals and jitter is measured by thesame method as the method used by the digital signal processing section60. The testing apparatus 100 measures jitter by using the comparator 20that outputs the digital signals of three values.

As shown in FIG. 7, the testing apparatus 100 that has a simplestructure as compared to the conventional method could perform themeasurement with less than 4% of measurement error from the conventionalmethod in the both signals-under-measurement having less noise and muchnoise.

FIGS. 8A and 8B show exemplary configurations of the band limitingsection 62. The band limiting section 62 of this example is used in thedigital signal processing section 60 shown in FIG. 5A. The band limitingsection 62 shown in FIG. 8A has a filter 74 and a Hilbert transformer76.

The filter 74 provides the discrete signal outputted out of the digitalsignal converting section 50 and passes frequency components to bemeasured. The Hilbert transformer 76 performs Hilbert transform on thedigital signal outputted out of the filter 74. For example, the Hilberttransformer 76 generates a signal from the digital signal whose phase isdelayed by 90 degrees. The band limiting section 62 outputs an analyticsignal wherein the digital signal outputted out of the filter 74 isregarded as a real part and the signal outputted out of the Hilberttransformer 76 is regarded as an imaginary part.

The band limiting section 62 shown in FIG. 8B has a filter 74 and mixers78 and 82. The filter 74 is the same one with the filter 74 shown inFIG. 8A. The mixers 78 and 82 provide the digital signals being splitand outputted out of the filter 74, perform quadrature modulation onthem and output them. For example, the mixers 78 and 82 provide carriersignals whose phases are different by 90 degrees and output bymultiplying the digital signals with the carrier signals. The bandlimiting section 62 outputs an analytic signal wherein the digitalsignal outputted out of the mixer 78 is regarded as a real part and thedigital signal outputted out of the mixer 82 is regarded as an imaginarypart.

The analytic signal having the frequency components to be measured ofthe signal-under-test may be generated by such configuration.

The filter 74 may pass components around the carrier frequency of thesignal-under-test among the frequency components of thesignal-under-test or may pass a frequency components of frequency bandcontaining no carrier frequency of the signal-under-test.

FIG. 9 is a graph showing one exemplary frequency band transmittedthrough the filter 74. As described above, the filter 74 passes bandcontaining no carrier frequency among the frequency components of thesignal-under-test. The carrier frequency component of thesignal-under-test is not a noise component and has large energy ascompared to the other frequency components. Therefore, when thecomponent of the carrier frequency is not removed, it will be requiredfor the apparatus to provide a measuring range and an arithmeticoperation range in which the energy of the carrier frequency may bedominant even though it is an unnecessary component in the measurementof noise. Therefore, it is unable to maintain sufficient resolution inthe arithmetic operation and others for noise components that have verysmall energy as compared to the components of the carrier frequency andhence it is unable to accurately measure the noise component.

As compared to that, the measuring apparatus 10 of this example canaccurately measure the noise component because it operates by removingthe carrier frequency component of the signal-under-test and byextracting the noise component to be measured. Preferably, the filter 74removes higher-order harmonic components of the carrier frequencycomponent.

FIG. 10 shows another exemplary configuration of the measuring apparatus10. The measuring apparatus 10 further includes a filter 75 in additionto the components of the measuring apparatus 10 shown in connection withFIG. 1. The filter 75 shown in FIG. 10 may have the same function withthe filter 74 shown in FIG. 8. The other components have the same orsimilar functions and configurations with the components denoted by thesame reference numerals and explained in FIG. 1.

The filter 75 in this example provides the signal-under-test outputtedout of the DUT 200, and passes the frequency components to be measuredto input to the comparator 20.

FIG. 11 is a chart showing one exemplary operation of the comparator 20and the strobe timing generator 30. In this example, the measuringapparatus 10 according to the present embodiment samples thesignal-under-test at the frequency of integer multiple of thegenerated-strobe frequency using the equivalent-time sampling approachby sequentially receiving the signal-under-tests and by shifting thephase of the strobe with respect to the signal-under-test. In thepresent embodiment, a case that the measuring apparatus 10 receives thesequential repetitions of the same signal-under-tests (asignal-under-test A and a signal-under-test B) twice will be described.

For the signal-under-test A, the strobe timing generator 30 generates astrobe signal A placed almost at equal time intervals synchronously (orasynchronously) with the period or test rate of the signal-under-test atfirst.

Here, the strobe timing generator 30 generates the strobe signal to beinputted to the comparator 20 based on a phase of a triggering signalsynchronized with the signal-under-test. For example, the strobe timinggenerator 30 starts to output the strobe signal A after an elapse of apredetermined offset time based on the triggering signal having thepredetermined phase with respect to the signal-under-test A.

Then, the strobe timing generator 30 starts to output the strobe signalB after an elapse of the predetermined offset time based on thetriggering signal in the same manner for the signal-under-test B to beprovided after the signal-under-test A. Strobes of the strobe signal Bare placed at the same time intervals with the strobe signal A.

Here, the phase of the triggering signal, which is the base timing ofthe strobe signal A, is almost same with the phase of the triggeringsignal, which is the base timing of the strobe signal B, and the strobeintervals of the strobe signal A is same with that of the strobe signalB. Still more, the offset time of the strobe signal A to the triggeringsignal and the offset time of the strobe signal B to the triggeringsignal may be different by about a half of the strobe interval. That is,when the strobe signal A is overlapped with the strobe signal B, thestrobe signals A and B are interleavingly placed almost at equalintervals.

It is then possible, by providing such strobe signals A and Bsequentially to a single comparator in order to sample thesignal-under-test equivalently at the frequency that is twice thegenerating frequency of the strobe signal The strobe timing generator 30may include an oscillation circuit for generating the strobe signal inwhich strobes are arranged at predetermined time intervals and a delaycircuit for delaying the output of the oscillation circuit for example.In this case, the oscillation circuit sequentially generates the strobesignals A and B. Then, the delay circuit sequentially delays therespective strobe signals corresponding to the offsets of the respectivestrobe signals.

Although this example has been explained by using the strobe signals Aand B, the strobe timing generator 30 may sequentially generate manymore strobe signals in another example. Equivalent time measurement maybe carried out at higher frequency by sequentially changing the offsetof those strobe signals.

FIG. 12 is a diagram showing another exemplary configuration of themeasuring apparatus 10. The measuring apparatus 10 of this examplefurther includes a clock regenerator 25 in addition to the components ofthe measuring apparatus 10 explained in connection with FIG. 1. Theother components are the same with those in the measuring apparatus 10explained in connection with FIGS. 1 through 11, so that theirexplanation will be omitted here. Based on the signal-under-test, theclock regenerator 25 generates a recovered clock synchronized with thesignal-under-test and inputs the recovered clock as a triggering signalto the strobe timing generator 30. Such configuration allows the timingfor starting to generate the strobe signals A and B explained in FIG. 11to be controlled and the strobe signals A and B having a predeterminedphase difference to be generated.

FIG. 13 is a diagram showing another exemplary configuration of thecomparator 20. The measuring apparatus 10 of this example has twocomparators 20-1 and 20-2 (hereinafter denoted generally as 20). Eachcomparator 20 is the same with the comparator 20 explained in FIG. 3A.Still more, the same first threshold voltage VOH and second thresholdvoltage VOL are fed to each comparator 20. Further, thesignal-under-test is split into two and inputted to the respectivecomparators 20. The measuring apparatus 10 may further include aninputting section 90 for splitting and inputting the signal-under-testto the respective comparators 20 in parallel. In this case, the strobetiming generator 30 inputs the strobe signals having different phases tothe respective comparators. For example, the strobe timing generator 30inputs the strobe signal A shown in FIG. 11 to the comparator 20-1 andinputs the strobe signal B shown in FIG. 11 to the comparator 20-2.Thereby, interleaving sampling may be carried out by using the twocomparators 20 and the signal-under-test may be measured by thefrequency by the frequency of twice of the strobe-signal generatingfrequency.

FIG. 14 is a chart showing one exemplary operation of the comparator 20and the strobe timing generator 30 shown in FIG. 13. The strobe timinggenerator 30 generates the strobe signal A (1, 2, 3, . . . ) and thestrobe signal B (A, B, C, . . . ) and inputs them to the respectivecomparators 20.

The capture memory 40 aligns and stores the comparison results of thetwo comparators 20 corresponding to the phase of the correspondingstrobe signals. For example, the capture memory 40 sequentially alignsand stores the comparison results corresponding to the strobe 1, strobeA, strobe 2, strobe B, . . . shown in FIG. 14. The strobe signals A andB are generated in the same time in such a case, so that it is notnecessary to generate the respective strobe signals based on thetriggering signal. It will do if a strobe groups in which the strobesignals A and B are superimposed are arranged almost at same timeintervals. For example, the strobe timing generator 30 may have acircuit for generating the strobe signal A and a circuit for generatingthe strobe signal B by delaying the strobe signal A.

Still more, although the case of having the two comparators 20 has beenexplained in this example, the measuring apparatus 10 may have morecomparators 20. In this case, it is possible to measure higher frequencyby changing the offset of the strobe signals to be inputted to therespective comparators 20.

However, the sampling method explained in connection with FIGS. 11through 14 may cause an error in the measured result if the phase ofeither one strobe signal is erroneous with respect to preset phase.Therefore, it is preferable to correct the phase error of the strobesignal, i.e., the measurement error based on the error of the samplingtiming.

FIGS. 15 and 16 are flowcharts showing one exemplary method forcorrecting errors of the sampling timing. This correction may be carriedout by the digital signal processing section 60. At first, the digitalsignal processing section 60 calculates an ideal value of phasedifference of the sampling timing of the respective data series sampledcorresponding to the respective strobe signals in an ideal phasedifference calculating step S300. For example, the phase difference maybe given by 2π(Δt/T), where the ideal value of the difference of theoffset of the respective strobe signals is Δt and the average period ofthe signal-under-test is T.

Next, the digital signal processing section 60 selects an arbitrary dataseries as a reference among the plurality of data series and calculatesa spectrum of the data series in a reference spectrum calculating stepS302. That spectrum may be obtained by performing a fast Fouriertransform on the data series.

Next, the digital signal processing section 60 selects a data seriesother than the reference data series and calculates a spectrum of thatdata series in a comparative spectrum calculating step S304. Thatspectrum may be also obtained by performing a fast Fourier transform.

Next, the digital signal processing section 60 calculates across-spectrum of the spectrum of the reference data series and that ofthe data series for comparison in a cross-spectrum calculating stepS306. This cross-spectrum may be obtained by complex multiplication ofcomplex conjugate spectrum of the spectrum of the reference data seriesand the spectrum of the data series for comparison.

Next, the digital signal processing section 60 calculates a phasedifference between the reference data series and the data series forcomparison in a phase difference calculating step S308. This phasedifference may be calculated based on the cross-spectrum calculated inStep S306. That is, a phase component of the cross-spectrum representsthe phase difference of the reference data series and the data seriesfor comparison.

Although the phase difference has been calculated by using thecross-spectrum of the two data series in Steps S304 and S306, the phasedifference may be calculated by another method. For example, the digitalsignal processing section 60 may calculate the phase difference based oncross-correlation function of the two data series.

Next, it is judged in Step S310 whether or not the digital signalprocessing section 60 has calculated the phase difference for all of thedata series to be compared. When there exists data series whose phasedifference from the reference data series is not calculated, the digitalsignal processing section 60 repeats the processes in Steps 304 through308 for that data series.

When the digital signal processing section 60 has calculated the phasedifference for all of the data series to be compared, the digital signalprocessing section 60 corrects the measurement error based on the phasedifference of the respective data series for comparison in an errorcorrecting step S312. For example, the digital signal processing section60 corrects the respective data series based on the difference of thephase difference of each data series for comparison from the ideal phasedifference found in Step 300.

FIG. 16 is a flowchart showing one exemplary process in the errorcorrecting step S312. At first, the digital signal processing section 60calculates the sampling timing error of the data series for comparisonbased on the phase difference between the reference data series and thedata series for comparison in a timing error calculating step S314. Thetiming error may be calculated based on the ideal phase difference.

Next, the digital signal processing section 60 judges whether or not thetiming error is greater than a predetermined threshold value in acomparing step S316. When the timing error is smaller than the thresholdvalue, the digital signal processing section 60 moves to the process inStep S320 without correcting the corresponding data series. When thetiming error is greater than the threshold value, the digital signalprocessing section 60 corrects the corresponding data series in acorrecting step S318. For example, the digital signal processing section60 may correct the data series by shifting the phase of spectrum of thatdata series based on the timing error.

Next, the digital signal processing section 60 judges whether or not thetiming error has been corrected for all of the data series. When thereexists data series whose timing error has not been corrected, thedigital signal processing section 60 repeats the processes from StepS314 to Step S318 to that data series. When the correction of the timingerror has been carried out for all of the data series, the digitalsignal processing section 60 generates the data series whose timingerrors have been corrected respectively in a data series generating stepS322. For example, it is possible to obtain the data series whose timingerror has been corrected by performing an inverse fast Fourier transformon the spectrum of each data series whose timing error has beencorrected.

Then, the digital signal processing section 60 aligns the respectivedata series in an aligning step S324. For example, the digital signalprocessing section 60 aligns the respective data corresponding tosampling timing of respective data.

The measurement error caused by the error of sampling timing may becorrected through such processes. Thereby, jitter may be measured moreaccurately.

FIG. 17 is a diagram showing another exemplary configuration of thetesting apparatus 100. The testing apparatus 100 of the presentembodiment further includes a function for performing a function test ofthe DUT 200 in addition to the function for performing the jitter testby the testing apparatus 100 explained in connection with FIGS. 1through 16.

In addition to the components of the testing apparatus 100 explained inconnection with FIGS. 1 through 16, the testing apparatus 100 of thisembodiment further includes a pattern generator 65 and a patterncomparing section 55. Still more, the judging section 70 has a logicjudging section 75 and a jitter judging section 77. The other componentshave the same or similar functions and configurations with thecomponents denoted by the same reference numerals and explained in FIGS.1 through 16.

In performing the functional test of the DUT, the pattern generator 65inputs a test signal having a predetermined data pattern to the DUT 200.The comparator 20 detects a data pattern of the signal-under-test bycomparing a voltage value of a signal-under-test outputted out of theDUT 200 with a predetermined threshold voltage at timing of given strobesignals.

While the strobe timing generator 30 generates the strobe signals atthis time, the strobe timing generator 30 generates the strobe signalscorresponding to a test rate synchronized with a period of thesignal-under-test in performing the functional test. For example, thestrobe timing generator 30 generates one strobe signal at almost-centertiming of each test rate. Thereby, the comparator 20 detects a datavalue in each period of the signal-under-test.

The strobe timing generator 30 may generate the strobe signalsindependent of the test rate in performing the jitter test as describedabove. The strobe timing generator 30 has the oscillation circuit forgenerating the strobe signals for example, and the operation of theoscillation circuit may be controlled by the test rate in performing thefunctional test and it is not necessary to be controlled by the testrate in performing the jitter test. Still more, the strobe timinggenerator 30 may have a first oscillation circuit for generating thestrobe signals in performing the functional test and a secondoscillation circuit for generating the strobe signals in performing thejitter test. At this time, the operation of the first oscillationcircuit is controlled by the test rate and the second oscillationcircuit is operated independently of the test rate.

In performing the functional test, the pattern comparing section 55compares the data pattern of the signal-under-test given by thecomparison results stored in the capture memory 40 whether or not itcoincides with a preset expected value pattern. The pattern generator 65may generate the expected value pattern based on the data pattern of thetest signal.

The logic judging section 75 judges whether or not the DUT 200 isdefect-free based on the comparison result in the pattern comparingsection 55.

The digital signal converting section 50, the digital signal processingsection 60 and the judging section 70 may be a computer with an embeddedsoftware. In this case, the testing apparatus 100 can perform the jittertest by using the conventional testing apparatus for testing functionswithout adding any hardware. Therefore, the test of the DUT 200 may beperformed at low cost.

FIG. 18 is a diagram showing one exemplary configuration of anelectronic device 400 according to another embodiment of the invention.The electronic device 400 has an operation circuit 410 for generatingthe signal-under-test and the measuring apparatus 10. The electronicdevice 400 may have a part of the configuration of the operation circuit410 and the measuring apparatus 10 within a package formed of such asresin and ceramic.

The operation circuit 410 operates corresponding to an external signalinputted from the outside for example and outputs the signal-under-test.The measuring apparatus 10 measures the signal-under-test outputted outof the operation circuit 410.

The measuring apparatus 10 may have a structure similar to that of themeasuring apparatus 10 explained in connection with FIGS. 1 through 16.For example, the measuring apparatus 10 may have the comparator 20 andthe capture memory 40. In this case, the comparator 20 is provided thestrobe signals explained in connection with FIGS. 1 through 16. Thestrobe signals may be given from the outside or may be generated withinthe electronic device 400.

It is preferable for the electronic device 400 to also have the strobetiming generator 30 when the strobe signals are to be generated withinthe electronic device 400. As explained in connection with FIGS. 1through 16, the capture memory 40 stores the measurement resultsobtained by sampling the signal-under-test equivalently at highfrequency.

Therefore, it is possible to accurately measure jitter in the electronicdevice 400 by accessing the comparison results stored in the capturememory 40. At this time, the external apparatus is not required tomeasure the high-speed signal-under-test, so that its equipment cost canbe reduced.

FIG. 19 is a diagram showing a strobe signal generated by the strobetiming generator 30. The strobe timing generator 30 according to thepresent embodiment may used for either of the measuring apparatus inFIG. 1 and the measuring apparatus in FIG. 10. The strobe timinggenerator 30 according to the present embodiment sequentially generatesstrobe signals at a period (T+Δ) different from a period (T) by apredetermined value (Δ). That is to say, the strobe timing generator 30generates strobe signals of which phase relative to that of thesignal-under-test is gradually changed. The signal-under-test accordingto the present embodiment is a signal indicative of approximately thesame waveform at the cycle (T).

Additionally, the strobe timing generator 30 according to the presentembodiment may generate a strobe signal at a cycle which does notsatisfy Nyquist theorem for the signal-under-test. That is, the strobetiming generator 30 according to the present embodiment undersamples thesignal-under-test. For example, the strobe timing generator 30 generatesa strobe signal at a cycle larger than half of that of thesignal-under-test. In the present embodiment, the strobe timinggenerator 30 generates strobe signals at a cycle larger than that of thesignal-under-test at even intervals as shown in FIG. 19.

As described above, by gradually changing the relative phase between thestrobe and the repetitive signal-under-test, the signal-under-test canbe equivalently sampled at high frequency.

For example, when the cycle of the signal-under-test is 400 ps and thecycle of the strobe signal is 405 ps, the phase of the strobe signalrelative to the signal-under-test is changed by 5 ps per cycle. Thewaveform for each cycle of the signal-under-test is approximately thesame, so that the signal-under-test can be equivalently sampled at thecycle of 5 ps.

The capture memory 40 may store the comparison result outputted by thecomparator 20 according to the strobe signal in chronological order. Thedigital signal converting section 50 retrieves the comparison result ofthe predetermined number of points among the comparison results storedin the capture memory, convert the same to digital signals and input thesame in the digital signal processing section 60.

For example, from the capture memory 40, the digital signal convertingsection 50 may retrieve the comparison results with the total number ofsamples, which is determined by satisfying the restriction ofcorresponding to the integer number of cycles of the signal-under-test.That is, the digital signal converting section 50 may obtain thedivision result by calculating (the period of thesignal-under-test)/(the difference between the period of the strobe andthe period of the signal-under-test), and retrieve data with the integermultiple of the division result from the comparison results beingcontinuously stored in the capture memory 40.

As described above, when the cycle of the signal-under-test is 400 psand the cycle of the strobe signal is 405 ps, the number of points ofthe comparison result corresponding to one cycle of thesignal-under-test is 160. In this case, the digital signal convertingsection 50 may retrieve the comparison result with the number of pointswhich is integer multiple of 160 as a discrete waveform from the capturememory 40. Therefore, a processing such as a Fourier Transform can beperformed without multiplying the discrete waveform by a window functionsuch as a Hanning window. Accordingly, a measurement with a highfrequency resolution can be achieved in comparison with the case thatthe window function is used. Additionally, the measurement time can beminimized. The discrete signal converting section 50 may retrieve themaximum number of points of the comparison result corresponding tointeger multiple cycles of the signal-under-test within the number ofpoints of the comparison result stored in the capture memory.

Each of FIG. 20 and FIG. 21 is a diagrams explaining an example ofoperation of the digital signal processing section 60. The digitalsignal processing section 60 performs a Fourier Transform on the digitalsignal inputted from the digital signal converting section 50 to convertthe same to a signal in the frequency domain. FIG. 20A shows an exampleof digital signal in the frequency domain.

At this time, when the number of points of data inputted from thedigital signal converting section 50 is power-of-two, the digital signalprocessing section 60 may perform a Fast Fourier Transform on theretrieved data. Alternatively, when the number of points of datainputted from the digital signal converting section 50 is notpower-of-two, the digital signal processing section 60 may perform aFourier Transform on the retrieved data with the mixed-radix algorithm.For example, when the number of points of data is power-of two, i.e. theradix is only two, the digital signal processing section 60 may performa one-dimensional Fast Fourier Transform. Additionally, when the numberof points of data is indicated by the product of a plurality of radixesor the mixed radix, the digital data processing section 60 may calculateby performing the mixed-radix FFT, the prime factor FFT or thesplit-radix FFT dependent on the number of points of the radix.

Next, the digital signal processing section 60 extracts the frequencycomponent around the carrier frequency of the signal-under-test. FIG.20B shows an example of extracted frequency component. FIG. 20B shows anexample of frequency component extracted when the carrier frequency ofthe signal-under-test is about 16 MHz, and the cut off frequency isabout 15 MHz+−5 MHz.

Next, the digital signal processing section 60 performs an inverseFourier Transform on the extracted frequency components to convert thesame to a signal in the time domain. FIG. 21A shows an example of signalin the time domain. By such processing, the analytic signal of thesignal-under-test can be obtained.

Next, the digital signal processing section 60 calculates theinstantaneous phase φ(t) of the signal-under-test based on the analyticsignal. Additionally, the digital signal processing section 60calculates the instantaneous phase noise Δφ(t) of the signal-under-testby removing the linear component from the instantaneous phase. Themethod of calculating the instantaneous phase noise Δφ(t) from theinstantaneous phase φ(t) is the same as the method shown in FIG. 6A.FIG. 21B shows an example of calculated instantaneous phase noise Δφ(t).

As described with reference to FIG. 6B, the digital signal processingsection 60 can calculate jitter of the signal-under-test based on thecalculated instantaneous phase noise Δφ(t). Here, the accuracy of thecalculated instantaneous phase noise Δφ(t) is changed depending on thedifference between the cycle of the signal-under-test and that of thestrobe signal, i.e. the time resolution.

FIG. 22 shows an example of instantaneous phase noise Δφ(t) calculatedwhen the cycle resolution (Δ) between the cycle of the signal-under-testand that of the strobe signal is changed. In the present embodiment, theinstantaneous phase noise for each of the cycle resolution (Δ) of thecycle 5 ps, 10 ps, 20 ps, and 40 ps is indicated. When the cycleresolution (Δ) is changed as shown in FIG. 22, the waveform of thecalculated instantaneous phase noise is changed. Therefore, it ispreferred that the cycle resolution (Δ) of the cycles is selected as avalue smaller than the jitter value of the signal-under-test, thestandard deviation and the rms value.

FIG. 23 is a diagram illustrating the measurement band width dependencyof the jitter value calculated for the cycle resolution (Δ). The jittervalue according to the present embodiment is calculated for the cycleresolution (Δ) when the rms value of the jitter variation included inthe signal-under-test is 2 ps. Here, the horizontal axis in FIG. 23 iscorresponded to the cut off frequency shown in FIG. 23.

FIG. 24 is a diagram showing an example of measurement error of thejitter value calculated for the cycle resolution (Δ) between each cycle.In the present example, the measurement value when the cycle resolution(Δ) is 5 ps is the true value.

As shown in FIG. 23 and FIG. 24, as the cycle resolution (Δ)-isincreased, the measurement error of the jitter value is rapidlyincreased.

The strobe timing generator 30 may set the cycle of the strobe signal soas to more reduce the cycle resolution (Δ). For example, when a pluralkinds of cycles can be selected as cycles of the strobe signal in thestrobe timing generator 30, the strobe timing generator 30 may select acycle for which the cycle resolution (Δ) is more reduced.

Additionally, the strobe timing generator 30 may set the cycle of thestrobe signal such that the difference between the cycle of the strobesignal and that of the signal-under-test is a value dependent on theamplitude of jitter to be measured or the time resolution forcalculating jitter. For example, if the amplitude value of the jitter tobe measured or the value of the time resolution of the jitter to becalculated is provided, the strobe timing generator 30 may sequentiallyset the cycle resolution of the strobe signals as being less than threetimes of the rms value of jitter or the value of the time resolution.Here, the jitter value to be measured may be the peak-to-peak value ofthe timing jitter. Additionally, it is preferred that the value of thecycle of the signal-under-test is provided to the strobe timinggenerator 30.

The maximum value of the difference between the timing for each edge ofthe signal-under-test and the ideal timing, i.e. timing jitter isdetermined by the amplitude value of the timing jitter. That is, theamplitude value of jitter makes the timing for each edge of thesignal-under-test deviate from the ideal timing. Therefore, the timingfor each edge of the signal-under-test may be changed within two-threetimes of jitter value around the ideal timing. Accordingly, the cycleresolution (Δ) of being less than two or three times of the jitter valuecan be used to accurately detect the timing jitter (the difference oftiming of the signal-under-test and its ideal timing) in thesignal-under-test.

Additionally, by setting the cycle resolution (Δ) be less than the timeresolution for calculating the jitter value, the jitter value at thetime resolution can be more accurately calculated.

FIG. 25 is a diagram showing an example of configuration of a patterngenerator 65 included in the testing apparatus 100 shown in FIG. 17. Thepattern generator 65 includes a pattern generator 67 for generating thesignal pattern of a test signal and a driver 69 for outputting the testsignal based on the test pattern. The driver 69 operates according to apredetermined test rate and makes the device under test 200 outputsignal-under-test at a cycle according to the test rate of the deviceunder test or integer multiple of the test rate. In the presentembodiment, the driver 69 receives timing signals at the cycle accordingto the test rate T and makes the device under test 200 outputsignal-under-test in accordance with the cycle

Meanwhile, the strobe timing generator 30 generates strobe signals at acycle t+Δ larger than a test rate T by a predetermined value. By suchoperation, a high-speed signal-under-test can be accurately measured ata low operational cycle speed. It is preferred that the strobe timinggenerator 30 has plural kinds of cycle resolutions Δ of the strobesignal which can be selected as the test rate T. For example, the timinggenerator 30 has plural sets of timings which can be set thereto, andthe cycle resolutions Δ of the test rate and that of the strobe signalwhen each timing being set may be measured in advance.

Among those timing set the strobe timing generator 30 may select atiming set at which the cycle resolution Δ is minimized, and also mayselect a timing set at which the cycle resolution Δ is less thantwo-three times of the jitter value to be measured.

FIG. 26 shows an example of instantaneous phase φ(t), linear phase andinstantaneous phase noise Δφ(t) of the signal-under-test described inFIG. 21. An upper diagram shows the instantaneous phase φ(t) and alinear phase 2πf₀t+φ₀ of the signal-under-test and a lower diagram showsthe instantaneous phase noise Δφ(t) of the signal-under-test. Thetesting apparatus 100 may calculates (φ₀|₂−φ₀|₁)/(2πf₀) based on thevalue of the instantaneous phase φ(t) of two signals at a predeterminedtime to obtain a deterministic skew between two signals. Here, thedeterministic skew is the different between electrical lengths of thepaths through which two signals are propagated The testing apparatus maycalculate (Δφ(t)|₂−Δφ(t)|₁)/(2πf₀) to obtain a random skew between twosignals.

For example, the testing apparatus 100 includes two comparators 20 inparallel. Signals are inputted to those comparators 20 at the same time.Additionally, the same strobe signals are provided to those comparators.That is to say, the testing apparatus 100 undersamples two signalsinputted to the comparators 20 at the same time.

Then, the testing apparatus 100 calculates the instantaneous phase φ(t)for each of the signals, and calculates the value for each of theinstantaneous phases φ(t) at a predetermined time (e.g. t=0). Therebythe difference between the estimated values indicates the deterministicskew. Such processing may be performed by the digital signal convertingsection 50 and the digital signal processing section 60. Additionally,the testing apparatus 100 may include two capture memories 40corresponding to two comparators 20. The digital signal convertingsection 50 receives data from two capture memories 40 and calculates theabove-described deterministic skew and random skew.

FIG. 27 is a diagram showing a comparison between the result ofmeasuring jitter by the testing apparatus 100 and the result ofmeasuring jitter by a jitter measuring apparatus E5052A. In FIG. 27, therange of the results of measured jitter values are shown by dottedlines. The jitter measuring apparatus may be a typical measurementapparatus having a function for measuring jitter.

Additionally, the resultant values of measured jitter by the testingapparatus 100 are indicated by circles and triangles in FIG. 27. Asshown in FIG. 27, the measurement results by the testing apparatus 100are well matched with the measurement results by the measuring apparatusused for measuring jitter. That is, it is indicated that the testingapparatus 100 can accurately measure jitter.

Although the invention has been described by way of the exemplaryembodiments, it should be understood that those skilled in the art mightmake many changes and substitutions without departing from the spiritand scope of the invention.

It is obvious from the definition of the appended claims that theembodiments with such modifications also belong to the scope of theinvention.

As it is apparent from the above description, the invention enables thejitter test of the device-under-test to be performed at low cost. Stillmore, it enables the timing jitter to be accurately measured because itallows the timing noise to be measured by separating the measurementfrom amplitude noise. It also allows the measurement to be performed atspeed higher than the maximum frequency of the strobe signal generatedby the strobe timing generator.

The Appendix is incorporated herein as a part of the specification byreference.

1. A measuring apparatus for measuring a signal-under-test, comprising:a comparator for sequentially comparing voltage values of thesignal-under-test with a threshold voltage value fed thereto at timingof strobe signals sequentially fed thereto; a strobe timing generatorfor sequentially generating the strobe signals placed almost at equaltime intervals; a capture memory for storing the comparison result ofthe comparator; and a digital signal processing section for calculatingjitter of the signal-under-test based on the comparison result stored inthe capture memory.
 2. The measuring apparatus as set forth in claim 1,wherein the strobe timing generator sequentially generates the strobesignals at a cycle different from that of the signal-under-test.
 3. Themeasuring apparatus as set forth in claim 2, wherein the strobe timinggenerator sequentially generates the strobe signals at a cycle largerthan half of that of the signal-under-test.
 4. The measuring apparatusas set forth in claim 2, wherein the strobe timing generatorsequentially generates the strobe signals such that the differencebetween the cycle of the strobe signal and that of the signal-under-testis an equivalent sampling interval dependent on a jitter value to bemeasured.
 5. The measuring apparatus as set forth in claim 4, whereinthe strobe timing generator provides the jitter value to be measured andsequentially generates the strobe signals such that the differencebetween the cycle of the strobe signal and that of the signal-under-testis less than N times of jitter value, where N is a positive integer. 6.The measuring apparatus as set forth in claim 2, wherein the strobetiming generator sequentially generates the strobe signals such that thedifference between the cycle of the strobe signal and that of thesignal-under-test is a value dependent on a time resolution determinedby the jitter value under test.
 7. The measuring apparatus as set forthin claim 6, wherein the strobe timing generator sequentially generatesthe strobe signals such that the time resolution for calculating jitteris provided, and the difference between the cycle of the strobe signaland that of the signal-under-test is less than the time resolution. 8.The measuring apparatus as set forth in claim 2, wherein the digitalsignal processing section divides the cycle of the signal-under-test bythe difference between the cycle of the strobe signal and that of thesignal-under-test retrieves continuous data with the number of pointswhich is integral multiple of the division result among the data of thecomparison results stored in the capture memory in chronological orderand calculates the jitter based on the retrieved data.
 9. The measuringapparatus as set forth in claim 8, wherein the digital signal processingsection calculates the jitter based on a result obtained by performing aFast Fourier Transform on the retrieved data when the number of pointsof the retrieved data is power-of-two, and calculates the jitter basedon a result obtained by performing a mixed radix Fourier Transform, aprime factor Fourier Transform and a split-radix Fourier Transform onthe retrieved data with algorithm.
 10. The measuring apparatus as setforth in claim 1, further comprising a digital signal converting sectionfor generating a digital signal in which each voltage value of thesignal-under-test is converted into a digital value whose absolute valuefalls within a range smaller than n (where, n is a real number) based onthe comparison result stored in the capture memory; wherein the digitalsignal processing section calculates the jitter of the signal-under-testbased on the digital signal.
 11. The measuring apparatus as set forth inclaim 1 or 10, wherein the comparator outputs comparison resultsdifferent from each other depending on whether or not the voltage valueof the signal-under-test is greater than the threshold voltage value.12. The measuring apparatus as set forth in claim 11, wherein thedigital value converting section converts the comparison resultrepresenting that the voltage value of the signal-under-test is greaterthan the threshold voltage value into a digital value of 1 and convertsthe comparison result representing that the voltage value of thesignal-under-test is smaller than the threshold voltage value into adigital value of −1.
 13. The measuring apparatus as set forth in claim 1or 10, wherein the comparator provides first one of the thresholdvoltage and second one of the threshold voltage whose voltage value islower than the first threshold voltage and outputs comparison resultsdifferent from each other depending on whether or not the voltage valueof the signal-under-test is greater than the first threshold voltage,whether or not the voltage value of the signal-under-test is smallerthan the first threshold voltage and greater than the second thresholdvoltage or whether the voltage value of the signal-under-test is smallerthan the second threshold voltage.
 14. The measuring apparatus as setforth in claim 13, wherein the digital signal converting sectionconverts the comparison result indicating that the voltage value of thesignal-under-test is greater than the first threshold voltage value intoa digital value 1, converts the comparison result indicating that thevoltage value of the signal-under-test is smaller than the firstthreshold voltage value and is greater than the second threshold voltagevalue into a digital value 0 and converts the comparison resultindicating that the voltage value of the signal-under-test is smallerthan the second threshold voltage value into a digital value −1.
 15. Themeasuring apparatus as set forth in claim 1 or 10, wherein thecomparator provides three or more different threshold voltage values andoutputs comparison results different from each other depending on avoltage range specified by two neighboring threshold voltages to whichthe voltage value of the signal-under-test belongs.
 16. The measuringapparatus as set forth in claim 1, wherein the strobe timing generatorgenerates the strobe signals placed almost at equal time intervalsindependently of operation periods of the measuring apparatus.
 17. Themeasuring apparatus as set forth in claim 1, wherein the strobe timinggenerator generates one of the strobe signals per operation period ofthe measuring apparatus.
 18. The measuring apparatus as set forth inclaim 1, wherein the strobe timing generator generates the plurality ofstrobe signals per operation period of the measuring apparatus.
 19. Themeasuring apparatus as set forth in claim 10, wherein the digital signalprocessing section comprises: a band limiting section for passingfrequency components to be measured of the digital signal; and a phasedistortion estimating section for calculating phase noise of the digitalsignal outputted out of the band limiting section.
 20. The measuringapparatus as set forth in claim 19, wherein the band limiting sectionconverts the digital signal into an analytic signal; and the phasedistortion estimating section has an instantaneous phase estimatingsection for generating an instantaneous phase signal representing theinstantaneous phase of the signal-under-test; and a linear phaseremoving section for removing a linear component of the instantaneousphase signal to calculate phase noise of the signal-under-test.
 21. Themeasuring apparatus as set forth in claim 19, wherein the phasedistortion estimating section comprises: a zero-crossing timingestimating section for estimating zero-crossing timing series of thesignal-under-test based on the digital signals outputted out of the bandlimiting section; and a linear phase removing section for removing alinear component of the zero-crossing timing series to calculate phasenoise of the signal-under-test.
 22. (canceled)
 23. The measuringapparatus as set forth in any one of claim 19-claim 21, wherein thefilter passes frequency components of frequency band containing nocarrier frequency of the signal-under-test among frequency components ofthe signal-under-test.
 24. The measuring apparatus as set forth in claim1, further comprising a plurality of comparators arranged in parallel;and an input section for inputting the signal-under-test into each oneof the plurality of comparators in parallel; the strobe timing generatorinputs the strobe signals whose phases are different to the respectivecomparators; and the capture memory aligns and stores the comparisonresults in the plurality of comparators in accordance to phases of thecorresponding strobe signals. 25-27. (canceled)
 28. A testing apparatusfor testing a device-under-test, comprising: a measuring apparatus formeasuring jitter of a signal-under-test outputted out of thedevice-under-test; and a jitter judging section for judging whether ornot the device-under-test is defect-free based on the jitter measured bythe measuring apparatus; wherein the measuring apparatus comprises: acomparator for sequentially comparing voltage values of thesignal-under-test with a threshold voltage value given thereto at timingof strobe signals sequentially fed thereto; a strobe timing generatorfor sequentially generating the strobe signals placed almost at equaltime intervals; a capture memory for storing comparison results of thecomparator; and a digital signal processing section for calculating thejitter of the signal-under-test based on the comparison results storedin the capture memory.
 29. The testing apparatus as set forth in claim28, wherein the strobe timing generator sequentially generates thestrobe signals at a cycle different from that of the signal-under-test.30. The testing apparatus as set forth in claim 29 further comprising adriver that operates according to a predetermined test rate, for causingthe device under test to output the signal-under-test at the cycleaccording to the test rate, the strobe timing generator sequentiallygenerates the strobe signals at a cycle larger than the test rate by apredetermined value.
 31. The testing apparatus as set forth in claim 28,further comprising a logic judging section for judging whether or not adata pattern of the signal-under-test determined by the comparisonresults stored in the capture memory coincides with a preset expectedvalue pattern.
 32. A measuring method for measuring a signal-under-testhaving a predetermined period, comprising: a comparing step ofsequentially comparing voltage values of the signal-under-test with agiven threshold voltage value at timing of strobe signals sequentiallygiven; a strobe timing generating step of sequentially generating thestrobe signals placed almost at equal time intervals; a storing step ofstoring comparison results of the comparator; and a digital signalprocessing step of calculating the jitter of the signal-under-test basedon the comparison results stored in the storing step.
 33. A testingmethod for testing a device-under-test, comprising: a measuring step ofmeasuring jitter of a signal-under-test outputted out of thedevice-under-test; and a jitter judging step of judging whether or notthe device-under-test is defect-free based on the jitter measured in themeasuring step; wherein the measuring step includes: a comparing step ofsequentially comparing voltage values of the signal-under-test with agiven threshold voltage value at timing of strobe signals sequentiallygiven; a strobe timing generating step of sequentially generating thestrobe signals placed almost at equal time intervals; a storing step ofstoring comparison results of the comparator; and a digital signalprocessing step of calculating the jitter of the signal-under-test basedon the comparison results stored in the storing step.
 34. A measuringapparatus for measuring a signal-under-test having a predeterminedperiod, comprising: a comparator for sequentially comparing a voltagevalue of the signal-under-test, a first threshold voltage value and asecond threshold voltage value given thereto at timing of strobe signalssequentially given thereto to output comparison results of the values; acapture memory for storing the comparison result of the comparator; anda digital signal processing section for calculating jitter of thesignal-under-test based on the comparison result stored in the capturememory.
 35. The measuring apparatus as set forth in claim 34, whereinthe digital signal processing section comprises: a Hilberttransformation pair generating section for transforming the comparisonresult into an analytic signal; an instantaneous phase estimatingsection for generating an instantaneous phase signal representing aninstantaneous phase of the signal-under-test based on the analyticsignal; and a linear phase removing section for removing a linear phaseof the instantaneous phase signal to present phase noise of thesignal-under-test.
 36. An electronic device for outputting asignal-under-test comprising: an operation circuit for generating thesignal-under-test; and a measuring apparatus for measuring thesignal-under-test wherein the measuring apparatus comprises: acomparator for sequentially comparing a voltage value of thesignal-under-test with a threshold voltage value given thereto at timingof strobe signals sequentially given thereto; and a capture memory forstoring the comparison result of the comparator.
 37. The electronicdevice as set forth in claim 36, further comprising a strobe timinggenerator for sequentially generating strobe signals arranged almost atequal time intervals.
 38. The electronic device as set forth in claim37, wherein the strobe timing generator sequentially generates thestrobe signals at a cycle different from that of the signal-under-testby a predetermined value.
 39. A measuring apparatus for measuring asignal-under-test, comprising: a first comparator for sequentiallycomparing voltage values of a first signal-under-test with a thresholdvoltage value fed thereto at timing of strobe signals sequentially fedthereto at a timing at which the strobe signals are sequentially fedthereto; a second comparator for sequentially comparing voltage valuesof a second signal-under-test with a threshold voltage value fed theretoat approximately the same time; a strobe timing generator forsequentially generating the strobe signals placed almost at equal timeintervals; a capture memory for storing the comparison result of thecomparator; and a digital signal processing section for calculating theinstantaneous phase for each of the first signal-under-test and thesecond signal-under-test based on the comparison result stored in thecapture memory and calculating a deterministic skew between the firstsignal-under-test and the second signal-under-test based on eachinstantaneous phase.
 40. The measuring apparatus as set forth in claim39, wherein the strobe timing generator sequentially generates thestrobe signals at a cycle different from the cycle of thesignal-under-test by a predetermined value.
 41. A measuring apparatusfor measuring a signal-under-test, comprising: a first comparator forsequentially comparing voltage values of a first signal-under-test witha threshold voltage value fed thereto at timing of strobe signalssequentially fed thereto at a timing at which the strobe signalssequentially fed thereto; a second comparator for sequentially comparingvoltage values of a second signal-under-test with a threshold voltagevalue fed thereto at approximately the same time. a strobe timinggenerator for sequentially generating the strobe signals placed almostat equal time intervals; a capture memory for storing the comparisonresult of the comparator; and a digital signal processing section forcalculating the instantaneous phase for each of the firstsignal-under-test and the second signal-under-test based on thecomparison result stored in the capture memory, calculating theinstantaneous phase noise for each of the first signal-under-test andthe second signal-under-test based on each instantaneous phase andcalculating a random skew between the first signal-under-test and thesecond signal-under-test based on each instantaneous phase noise. 42.The measuring apparatus as set forth in claim 40, wherein the strobetiming generator sequentially generates the strobe signals at a cycledifferent from the cycle of the signal-under-test by a predeterminedvalue.